Question

(3.25) Calculate the molar mass of a compound if 0.372 mole of it has a mass of 152 g. (Hint: if 0.372 mole is to 152 g, how many grams will 1 mole have if you set this equation by ratio & proportion)

Caffeine is found in your coffee and your tea. Its molecular formula is C₈H₁₀N₄O₂. (a) Calculate the molar mass of caffeine. (b) Calculate the percent composition by mass of C, H, N, and O in caffeine. (c) Red bull contains 80 mg of caffeine. How many molecules of caffeine are there in red bull ? [C 12.011 amu; H 1.008 amu; N 14.008 amu; O 15.999 amu]

Answer 1

molar mass = (given mass)/(number of mole)
= 152/0.372
= 409 g/mol

a)
molar mass of C8H10N4O2 = (8*12.011)+(10*1.008)+(4*14.008)+(2*15.999)
= 96.088+10.08+56.032+31.998
= 194.198 amu
b)
%mass = {(mass of element)/(total mass)}*100
%mass of C = (96.088/194.198)*100
= 49.5%
%mass of H = (10.08/194.198)*100
= 5.2%
%mass of N = (56.032/194.198)*100
= 28.8%
%mass of O = (31.998/194.198)*100
= 16.5%

c)
given mass = 80 mg
= (80*10^-3) g
number of mole of caffeine = (given mass)/(molar mass)
= (80*10^-3)/(194.198)
= (0.412*10^-3) mole